Recently, a Multiple Input Multiple Output (MIMO) system has been spotlighted as a wideband wireless mobile communication technology. The MIMO system refers to a system capable of improving data communication efficiency using multiple antennas. The MIMO system may be classified into a space multiplexing scheme and a space diversity scheme, depending on whether or not the same data is transmitted.
The space multiplexing scheme refers to a scheme for simultaneously transmitting different data via a plurality of transmission antennas so as to rapidly transmit data without increasing system bandwidth. The space diversity scheme refers to a scheme for transmitting the same data via a plurality of transmission antennas so as to obtain transmission diversity. Examples of such a space diversity scheme include a space time channel coding scheme.
In addition, the MIMO technology may be classified into an open loop scheme and a closed loop scheme according to feedback of channel information from a reception side to a transmission side. The open loop scheme includes a Space-Time Trellis Code (STTC) scheme in which a transmission side transmits information in parallel, and a reception side detects a signal using a Zero Forcing (ZF) scheme and a Minimum Mean Square Error (MMSE) scheme repeatedly and obtains transmission diversity and coding gain using a space domain and Bell Laboratories Layered Space-Time (BLAST) for increasing information amount by the number of transmission antennas, or the like. The closed loop scheme includes a Transmit Antenna Array (TxAA) scheme, or the like.
A coordinated multi-point (CoMP) system (hereinafter, referred to as a CoMP system) is a system for improving throughput of a user located at a cell boundary by applying improved MIMO transmission in a multi-cell environment. If a CoMP system is applied, it is possible to inter-cell interference in a multi-cell environment. If such a CoMP system is used, a user equipment (UE) can commonly receive data from a multi-cell base station.
In addition, each base station simultaneously supports one or more UEs UE 1, UE 2, . . . , and UE K using the same radio frequency resources, thereby improving system performance. In addition, the base station may perform a space division multiple access (SDMA) method based on channel state information (CSI) between the base station and the UE.
Such a CoMP scheme may be divided into a coordinated MIMO joint processing (JP) scheme through data sharing and a coordinated scheduling/beamforming (CS/CB) scheme.
FIG. 1 is a conceptual diagram of Coordinated Multi-Point (CoMP) of intra base stations (BS) (or eNBs) and inter BSs (eNBs).
Referring to FIG. 1, intra BSs 110 and 120 and an inter BS 130 are present in a multi-cell environment. In a Long Term Evolution (LTE) system, the intra BS is composed of several cells (or sectors). Cells belonging to BSs to which specific UEs belong correspond to the intra BSs 110 and 120 for the specific UEs. That is, cells which share the same BSs as cells to which UEs belong correspond to intra BSs 110 and 120 and cells belonging to different BSs correspond to the inter BS 130. Cells based on the same BSs as specific UEs may exchange information (e.g., data, channel state information (CSI)) via an x2 interface, but cells based on different BSs may exchange inter-cell information via a backhaul 140.
As shown in FIG. 1, a single-cell MIMO user 150 located in a single cell may communicate with one serving BS in one cell (sector), and a multi-cell MIMO user 160 located at a cell boundary may communicate with a plurality of serving BSs in multiple cells (sectors).
If such a CoMP system is used, a UE may commonly receive data from multi-cell BSs (eNBs). That is, in a multi-cell environment, communication performance of a UE located at a cell boundary can be improved using the CoMP operation. When UEs belonging to cells which perform a CoMP operation perform multi-user-MIMO (MU-MIMO), CoMP sets may overlap. Due to such a problem, UEs belonging to different cells for performing CoMP MU-MIMO may not be aware of which reference signals are transmitted by BSs for performing the CoMP operation. If UEs are not aware of the reference signals of the BSs for performing the CoMP operation, communication performance may be deteriorated when CoMP MU-MIMO is performed.